I too don't really think height in and of itself will be a determining factor in a horse's smoothness, or lack there of, in gaits. I've ridden horses ranging from 13 hh, to 17 hh...and there have been plenty of smooth horses in that bracket, no matter how 'short' they were. It's conformation that will determine the gait; an upright shoulder, will give a horse a more rough gait, because he won't be able to reach foward as well. Same with posty legs, won't matter how tall the horse is, he may have a hard gait. There are just so many factors that have to be factored in, other than how tall a horse is.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."